1. Field of Invention
The present invention relates to a spread illuminating apparatus used as front-face illuminating means for various reflection-type display units and so on, and more particularly to a spread illuminating apparatus used as illuminating means for a liquid-crystal display unit.
2. Background of Related Art
A liquid crystal display unit that operates with a low electric power consumption increases in demand as a display unit which is mainly used in association with a computer since it has the features such as a thin-type, light-weight, etc. Liquid crystal which is a structural element of the liquid crystal display unit requires illuminating means for illuminating an image, which is different from a light-emission type element such as a CRT, because the liquid crystal does not emit light by itself. In particular, in demand for thinning the apparatus in recent years, there is frequently used a spread illuminating apparatus of a thin-plate-like side light type (light-conductive plate type) as illuminating means for irradiating the liquid crystal display unit.
Hereinafter, the structure of a spread illuminating apparatus 41 of the side light type will be described referred to FIG. 15.
Reference numeral 42 denotes a linear light source lamp such as a cold cathode fluorescent tube (CCFL) or a heat cathode fluorescent tube (HCFL) which is used as a light source of the spread illuminating apparatus. A transparent substrate 43 made of a material high in transmittance is shaped in a thin plate which is nearly rectangular in section. The light source lamp 42 is disposed at a given interval so as to be separated from and along one side end surface 44 thereof. For the purpose of illuminating the apparatus, the transparent substrate 43 may be shaped in a so-called wedge so as to be made gradually thinner as it is far from the one side end surface 44 along which the light source lamp 42 is disposed.
A lamp reflector 45 which is formed by evaporating silver or the like on a film is disposed on a portion, which is not opposite to the one side end surface 44, of the peripheral surface of the light source lamp 42. The provision of the lamp reflector 45 enables most of the light emitted from the light source lamp 42 to enter the interior of the transparent substrate 43 from the one side end surface 44. In addition, in order to prevent the leakage of the light, a reflection material 47 formed of a reflection tape or the like is added to side surfaces except for the one side end surface 44 of the transparent substrate 43 (only the side end surface 46 which is an opposite surface of the one side end surface 44 is shown in FIG. 15).
A light scattering pattern 49 (which will be described in detail later) is formed on a back surface 48 (lower side of FIG. 15) of the transparent substrate 43 in order to allow a light to be uniformly emitted from a screen of the spread illuminating apparatus unit without being influenced by a distance from the light source lamp 42, and a reflector 50 is disposed so as to cover the entire surface of the back surface 48 on the lower portion of the light scattering pattern 49. The reflector 50 allows a light which is supposed to be emitted from the back surface 48 of the transparent substrate 43 to be reflected thereby and to progress toward the front surface 51 of the transparent substrate 43 (toward an upper portion of FIG. 15).
Further, a diffusing plate 52 is disposed in the spread illuminating apparatus 41 so as to cover the entire front surface 51 of the transparent substrate 43. In order to avoid a phenomenon (a so-called dot image) that only the pattern of the light scattering pattern 49 is observed brightly because most of the light which progress in the transparent substrate 43 and is emitted from the front surface 51 is reflected at the light scattering pattern 49, the diffusing plate 52 is disposed, thereby a light passed through the diffusing plate 52 is superimposed on each other (that is, the light is diffused) to make the density and the emission distribution of a light substantially uniform to realize the uniform emission of the light on the screen.
The light scattering pattern 49 shown in FIG. 16 is formed in such a manner that the diameter of dots gradually increases from the one side end surfaces 44, along which the light source lamp 42 is disposed, toward the side end surfaces 46 which is an opposite surface of the one side end surface 44, for example, as disclosed in Japanese Patent Laid-open Publication No. Hei 5-134251. The light scattering pattern 49 is formed by coating a light diffusion-reflection material directly on the back surface 48 of the transparent substrate 43 by the screen printing system.
In this way, the light scattering pattern 49 allows the amount of light reflected thereby and emitted from the front surface 51 to change because the light scattering pattern 49 is formed by changing a rate of the light diffusion-reflection material per unit area depending on its location (hereinafter, the rate of predetermined material per unit area being called "area density"). That is, the light becomes higher in luminance as it is near the light source lamp 42. Therefore, in order to realize the uniform spread light emission, the light scattering pattern 49 is formed in such a manner that the area density of the light scattering pattern 49 increases more as it is far from the light source lamp 42 with the result that the amount of light reflected on the front surface 51 side increases more as it is far from the light source lamp 42. Therefore, taking the distance from the light source lamp 42 and the amount of light reflected by the light scattering pattern 49 into consideration, the light source lamp 42 is adapted to uniformly emit the light as a whole. The light scattering pattern 49 is indicated by oblique lines in FIG. 16 for the facilitation of understanding although it is not of a section.
The above-described light scattering pattern 49 is structured to coat the light diffusion-reflection material on the back surface 48 of the transparent substrate 43. However, since it is essential that the pattern 49 have a function of increasing the amount of reflected light, the pattern 49 may be designed in such a manner that a fine-concave/convex surface is formed directly on the back surface 48 of the transparent substrate 43, and the light is diffused or reflected by the concave/convex surface, for example, as disclosed in Japanese Patent Laid-open Publication No. Hei 9-33923.
Subsequently, a description will be given of the structure of an illuminating apparatus different from the above-described spread illuminating apparatus 41, which is particularly used as an auxiliary illuminating apparatus of the reflection type liquid crystal display device which is the illuminating means of the liquid crystal display device. Since the reflection type liquid crystal display device is structured in such a manner that the surrounding light is employed as illumination so that it can be irradiated on the screen when the surroundings are bright, no illuminating means is required inside of the device. However, in a case where the surroundings are relatively dark, since there occurs a drawback that it is difficult to observe an image, the auxiliary illuminating apparatus is required.
As an appropriate auxiliary illuminating means of the reflection type liquid crystal element of this type, there is disclosed, in Japanese Patent Application No. Hei 9-347648, a spread illuminating apparatus disposed on a front surface (screen side) of the reflection type liquid crystal element.
A transparent and spread illuminating apparatus 1' shown in FIG. 17 is disposed so as to cover the observation face F of the reflection type liquid crystal element L thus structured in use, and its structure is that a linear light source lamp 4 is disposed so as to be close to one side end surface 3 of the flat transparent substrate 2 which is made of a material high in transmittance and shaped in a rectangle in section as shown in FIGS. 17 and 18. The light source lamp 4 is formed of a cold cathode fluorescent tube (CCFL), a heat cathode fluorescent tube (HCFL) or the like.
Further, for the purpose of reducing weight of the apparatus, the transparent substrate 2 may be formed in a wedge-shaped so as to be made gradually thinner as it is far from the one side end surface 3 along which the light source lamp 4 is disposed.
In this example, it is assumed that, in FIG. 17, one surface (a lower side in FIG. 18) of the transparent substrate 2 on which the reflection type liquid-crystal element L abuts is a lower surface 5, and its opposite surface (an upper side in FIG. 18) that is on an observation face (screen) side is a top surface 6.
A light reflection pattern 7 is formed on the top surface 6 of the transparent substrate 2. The light reflection pattern 7 is made up of a large number of grooves 8 which are substantially triangular in section and a large number of flat portions 9 adjacent to the grooves 8.
The light reflection pattern 7 is designed in such a manner that intervals between which the grooves 8 are defined are different depending on position so that the brightness becomes nearly uniform at any positions of the transparent substrate 2 without being influenced by the distances from the light source lamp 4 as shown in FIG. 18. In other words, the ratio of the width (occupied area) of the grooves 8 to the width (occupied area) of the flat portions 9 is so set as to gradually increase more as far from the one side end surface 3 of the transparent substrate 2.
With the addition of the transparent and spread illuminating apparatus 1' thus structured to the reflection type liquid crystal element L as an auxiliary illumination, a light emitted from the light source lamp 4 is made incident to the interior of the transparent substrate 2 from the one side end surface 3 of the transparent substrate 2, and progresses toward the opposite surface 10 while the light repeats reflection and refraction in the interior of the transparent substrate 2. During this action, the light is emitted from the lower surface 5 of the transparent substrate 2 little by little with the result that the light is irradiated on the reflection type liquid crystal element L which is disposed in close contact with the transparent substrate 2. In addition, since the light reflection pattern 7 is formed on the transparent substrate 2, the the distribution of amount of light emitted from the lower surface 5 can become nearly uniform.
Although being omitted in FIGS. 17 and 18, since a peripheral surface of the light source lamp 4 which is not faced on the one side end surface 3 is covered with a film-shaped reflection member, the coupling efficiency of a light can be enhanced. Furthermore, if the side surfaces of the transparent substrate 2 except for the one side end surface 3 are also covered with a reflection member, since the light is prevented from being emitted from the side end surfaces, the amount of light from the lower surface 5 of the transparent substrate 2 can be increased. In particular, in the opposite surface 10 of the one side end surface 3, since the amount of emitted light is more than those of two other side surfaces, it is desirable that the opposite surface 10 is covered with a reflection member.
Also, a direction of the light emitted from the lower surface 5 of the transparent substrate 2 varies as an angle of reflection of the light changes in accordance with the configuration of the grooves 8 of the light reflection pattern 7. As a result, the configuration of the groove 8 can be appropriately set so that a large amount of light is emitted in a direction perpendicular to the lower surface 5 (that is, a front-surface direction).
By the way, because both of the above-described spread illuminating apparatuses 41 and 1' used as the illuminating means of the liquid crystal display device (also including the reflection type liquid crystal element L) use the cold cathode fluorescent tube (CCFL), the heat cathode fluorescent tube (HCFL) or the like as a light source, they suffer from problems stated blow.
That is, the above fluorescent tubes used as the light source lamps 4 and 42 have a tendency that their diameters are extremely reduced to satisfy the needs for thinning the apparatus recently, accordingly, since those tubes might be broken by even small impact, its handling must be done carefully.
In addition, in order to allow the fluorescent tubes used as the light source lamps 4 and 42 to emit a light, because a high voltage of several hundreds to 1000 V or higher is generally required, the fluorescent tubes are provided with a complicated illuminating circuit which is so-called inverter. For this reason, a space in which the inverter is located must be always ensured, and a demand for reducing the space where the inverter occupies increases in the needs for thinning and illuminating the apparatus. Also, there arises such a problem that a complicated countermeasure is required from the viewpoint of safety in high voltage.
If the fluorescent tube (linear light source) having the above problem is replaced by a dot-shaped light source such as an electric bulb or a light emission diode as the light source lamps 4 and 42, the above problem can be eliminated. However, in the case where the dot-shaped light sources are merely disposed on positions at which the light source lamps 4 and 42 of the above conventional spread illuminating apparatuses 41 and 1' are disposed, there arises such a problem that only a portion in the vicinity of the dot-shaped light source becomes bright with the result that uniformly spread light emission all over the top surface 6 cannot be realized.